2016 THEMIS SCIENCE NUGGETS

Evidence of kinetic Alfvén eigenmode in the near-Earth magnetotail during substorm expansion phase

by Suping Duan
State Key Laboratory of Space Weather
National Space Science Center, Chinese Academy of Sciences

Introduction

Kinetic Alfvén waves (KAW) occur in a wide variety of plasmas. Theoretically, KAW is the extension of the shear Alfvén wave branch into the regime of small transverse wavelength in the intermediate-high beta plasma. With corrections from the finite ion gyroradius effect in direction perpendicular to the background magnetic field, ΔE⊥/ΔB⊥ for KAW is modified to be larger than the Alfvén speed. This feature of E/B is distinctly different from the MHD shear Alfvén waves and frequently used to identify KAW. The formation of KAW eigenmode is especially important for processes in the thin (~ion gyroradius) plasma sheet during substorms. One possible application of KAW eigenmode is on Hall fields and currents in the magnetotail reconnection. The Hall fields and current in reconnection have been explained in terms of KAW/ Alfvén eigenmode of the plasma sheet. Supporting this scenario, here we present the first THEMIS observation of unipolar KAW pulses identified as earthward propagating KAW eigenmodes in the NEPS at 12 R_E during substorm expansion phase. The polarity of the observed KAW eigenmode is exactly consistent with that of Hall field/current in reconnection. The field-aligned current and Poynting flux carried by KAW eigenmode may be an important link from magnetotail reconnection to the aurora.

Figure 1. The scheme of the close relation between KAW eigenmode and Hall field/current in magnetotail reconnection.

Results

Unipolar pulses of Kinetic Alfvén wave (KAW) are first observed in the near-Earth plasma sheet (NEPS) associated with dipolarizations during substorm expansion phases on 3 February 2008 and 7 February 2008 by THEMIS. The unipolar pulses were located at a trough-like Alfvén speed profile in the northern plasma sheet at a distance of 10-11 RE from Earth. The dominant wave components consist of a southward δEz toward the neutral plane and a +δBy toward the dusk (Figure 2c and 2d). The │δEz│/│δBy│ ratio was in the range of a few times of the local Alfvén speed (Figure 2e), a strong indication of KAW nature. The wave Poynting flux was earthward and nearly parallel to the background magnetic field (Figure 2g). These observational facts strongly indicate a KAW eigenmode that is confined by the plasma sheet but propagates earthward along the field line. The observed polarity of the KAW field/current is consistent with that of the Hall field/current in magnetic reconnection, supporting the scenario that the Hall fields/current propagate out from reconnection site as KAW eigenmodes (Figure 1).

Figure 2. Electron energetic particle flux, pitch angle distribution and electromagnetic field fluctuations during substorm expansion phase are observed by THD during the period of 0454-0500 UT on 3 Feb 2008 in the NEPS. From top to bottom: (a)electron energetic particle flux, (b)energetic electron pitch angle distribution, (c) the perturbations of three components of the electric field ,δE_x (blue line), δE_y (green line), δE_z (red line), (d) two components of the perturbation magnetic field, B_y (green line) and B_z (red line), (e) Local Alfvén speed, VA (black line), predicted propagation speed of KAW eigenmode, V_A* (red line) and the ratio of the perturbation electric (δE_z ) to magnetic (δB_y) fields, V_δEB (green line), (f) three components of Poynting flux, S_x (blue line), S_y(green line), S_z(red line), the parallel and total Poynting flux, (g) S_par (blue line) and S_t (pink line), respectively.

Aurora images on the footprint of THEMIS spacecraft suggest that KAW eigenmode may power aurora brightening during substorm expansion phase (Figure 3).

Figure 3. Auroral images from THEMIS ground station GILL and SNKQ during the time intervals 04:56:30 UT--04: 56:48 UT and 04:58:39 UT--04:58:57 UT on 3 February 2008. The probes' footprints (THD: triangle; THE: asterisk) are illustrated in each right auroral image.

Conclusion

We presented detailed THEMIS observations of unipolar pulses that are identified as KAW eigenmode in the near-Earth (10-11R_E) magnetotail during substorm expansion phase. The polarity of the KAW eigenmode is consistent with that of Hall fields and current at the Earthward side of the magnetotail reconnection. The KAW eigemode (and associated dipolarization) may lead to aurora brightening during substorm expansion phase.

Reference

Duan S. P., L. Dai, C. Wang, J. Liang, A. T. Y. Lui, L. J. Chen, Z. H. He, Y. C. Zhang,and V. Angelopoulos, Evidence of kinetic Alfvén eigenmode in the near-Earth magnetotail during substorm expansion phase, J. Geophys. Res. Space Physics, 121, 4316–4330, doi:10.1002/ 2016JA022431, 2016.

Dai, L. (2009), Collisionless magnetic reconnection via Alfvén eigenmodes, Phys. Rev. Lett., 102, 245003, doi:10.1103/PhysRevLett.102.245003. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.102.245003

Biographical Note

Suping Duan is an Associate Professor in space physics at the National Space Science Center (NSSC), Chinese Academy of Sciences (CAS). Her current research activities are focused on kinetic Alfvén waves in the near-Earth plasma sheet during substorms.

Please send comments/suggestions to

Emmanuel Masongsong / emasongsong @ igpp.ucla.edu